Most modern wind turbines are controlled and regulated continuously most often with the purpose of ensuring maximum power extraction from the wind under the current wind, weather, while at the same time ensuring that the loads on the different components of the wind turbine are at any time kept within acceptable limits. Desirably, the wind turbine may also be controlled to account for fast local variations in the wind velocity—the so-called wind gusts, and take into account the dynamic changes in the loads on the individual blades due to e.g. the passing of the tower or the actual wind velocity varying with the distance to the ground (the wind profile).
To this purpose a number of parameters are collected and monitored by the controllers in a wind turbine, such as for instance the current wind speed and direction, the wind shear and turbulence, the rotational speed of the rotor, the pitch angle of each blade, the yaw angle, information on the grid system, and measured parameters (e.g. stresses or vibrations) from sensors placed e.g. on the blades, the nacelle, or on the tower.
Based on these and following some control strategy the optimal control parameters of the turbine in order to perform optimally under the given conditions are determined. The methods of controlling the current performance, and thereby the power production and the load situation of the wind turbine, include for instance pitching of the blades, adjusting any different active aerodynamic devices for changing the aerodynamic surfaces of the blades such as flaps or vortex generating means, adjusting the power, and/or adjusting the rotational speed of the rotor. These parameters are here and in the following referred to as controllable parameters.
Sudden or abrupt changes in the wind conditions such as drops in the wind speed and large wind direction changes etc., may,—if the control strategy of the wind turbine is not determined or executed fast enough,—result in very high and unacceptable loads and moments in some of the components of the wind turbines, e.g. in the tower due to an undesirable pitching of one or more of the blades, or in the gears due to erroneous adjustments of the power. Such loads may be of considerable sizes and may in the worst case scenario in the extreme situations or over time lead to fatal damage of the turbine. Irrespective that the probability for such extreme situations to arise may be minimal, the possible implications are unacceptable, creating the need for fail-safe control methods capable of preventing these possibly rare but extreme eventualities.
Such wind load cases therefore need to be considered in the design and in the continuous control of the wind turbine to assure its structural integrity. Examples of extreme wind load conditions are prescribed in the IEC 61400 code and comprise among others the Extreme Operating Gust (EOG) load case, the Extreme Coherent gust with Direction change (ECD) load case, the Extreme Direction Change, the Extreme Coherent Gust (ECG), and the Extreme Wind Shear (EWS) load case.
Known controlling systems comprise using a measured or estimated wind speed and/or detecting the wind direction using a wind vane directly to determine the controllable parameters. However, such systems have in some situations and especially in extreme wind conditions turned out to be too slow or too inaccurate to detect the wind load condition changes in due time for the turbine to perform optimally, and may therefore be insufficient and in some cases even inadequate to protect the turbine.